Humanizing the mdx mouse model of DMD: the long and the short of it

Abstract

Duchenne muscular dystrophy (DMD) is a common fatal heritable myopathy, with cardiorespiratory failure occurring by the third decade of life. There is no specific treatment for DMD cardiomyopathy, in large part due to a lack of understanding of the mechanisms underlying the cardiac failure. Mdx mice, which have the same dystrophin mutation as human patients, are of limited use, as they do not develop early dilated cardiomyopathy as seen in patients. Here we summarize the usefulness of the various commonly used DMD mouse models, highlight a model with shortened telomeres like humans, and identify directions that warrant further investigation.

Fig. 1

Components of the dystrophin-associated glycoprotein complex. The dystrophin-associated glycoprotein complex (DGC) and proteins that interact with DGC are depicted here. Extracellular, membrane, cytoplasmic and nuclear components are shown. The proteins targeted in double knockout mouse models of DMD, as outlined in the text, are indicated: (1) Dystrophin (2) Utrophin (3) α-Dystrobrevin (4) α7-Integrin (5) Myogenic differentiation factor 1(Myod) (6) Glycans and (7) telomere length. The domains of dystrophin are also indicated: (i) N-terminal domain (ii) middle body domain, which includes the nNOS binding sites (iii) cysteine-rich domain and (iv) C-terminal domain. In addition, DMD disease modifiers not directly involved in the DGC but that are also targeted in double knockout studies are also highlighted

Fig. 2

Crossing scheme for generation of the mdx^4cv/mTR^G2 model of Duchenne Muscular Dystrophy. Breeding begins with an initial cross of a male mTRHet heterozygous animal with a female mdx^4cv/4cv homozygous animal, both of which are available live. The male progeny of this cross are bred with female mdx^4cv/4cv animals. It should be noted that because the dystrophin gene is X-linked, males are inherently hemizygous while females are homozygous. Progeny of the mdx^4cv/mTR^Het x mdx^4cv/4cv cross are denoted as “Het”. Male Het animals are heterozygous for the mTR mutation and have the mdx^4cv mutation on the X-chromosome. Given the one functional copy of Terc (mTR), Hets with normal telomere lengths are used as controls. To create the first generation of mdx4^cv/mTR^KO (designated as mdx^4cv/mTR^G1) animals with shortened telomeres, inter-cousin breeding of mdx^4cv/mTR^Het is performed to avoid genetic drift.^, Finally, mdx^4cv/mTR^G1 animals are crossed, again through inter-cousin breeding, to generate the second generation mdx^4cv/mTR^G2 animals. mdx^4cv/mTR^G2 animals have “humanized” telomere lengths that allow the full penetrance of skeletal and cardiac muscle phenotypes

Fig. 3

mdx^4cv/mTR^G2 animal model faithfully recapitulates the skeletal and the cardiac phenotype of DMD. Molecular and functional characterization of skeletal muscle and cardiac phenotypes in the mdx^4cv/mTR^G2 double knockout mouse. Overall, mdx^4cv/mTR^G2 exhibited decreased life span. In the skeletal muscle, mdx^4cv/mTR^G2 exhibited decreased exercise capacity, decreased muscle strength, histological evidence of muscular dystrophy, and kyphosis. In the heart, mdx^4cv/mTR^G2 displayed cardiac dysfunction as measured by MRI, histology, ECHO, ECG, and electron microscopy. Figure adapted from prior mdx^4cv/mTR^G2 studies^,